The present invention concerns a level measurement device with an adjustable measurement head and such a measurement head, as well as a level measurement method.
Level measurement devices, like, radar and ultrasonic sensors or optical systems, are used to determine the level of a product in a container or in the open. The shapes of the containers can vary sharply. In order to be able to use a measurement head in conjunction with various container geometries, it is known to provide them with a swivel mount that can be mounted at an appropriately selected location on the container wall, making it possible to establish the alignment of the transmitter/receiver unit of the measurement head, so that a satisfactory, reliably evaluable echo signal is obtained, if possible, under all operating conditions.
This type of measurement head has various drawbacks. In the first place, the quality of the measurements that can be obtained with such a measurement head depend strongly on how skillfully or unskillfully the incorporation position and alignment of the measurement head on the container were chosen. An improperly selected incorporation position can mean that useful measurements are only possible at certain filling levels. Generally, this means that extremely low levels or regions with interfering echoes are excluded from the useful measurement interval. However, it is precisely the extreme level values whose recording is particularly important, in order to avoid overfilling or emptying of the container. One can objectively assume that, in a bulk product container, a central, downward oriented mounting of the measurement head should be optimal, in order for the scanning signal emitted by the measurement head to always encounter the filling level and not be scattered on the container walls. This type of mounting, however, means that the measurement head lies directly opposite the tip of a product cone, which forms during filling of the container via a filling connector arranged in the center for technical reasons, so that, with a high filling level, the distance between the product and measurement head falls short of a minimum distance necessary for correct processing of the reflected echo signal. In addition, this type of scanning signal is very strongly influenced by the product stream during filling of the container, so that, precisely in this critical phase, a reliable measurement is not possible. An off-center, downward oriented mounting of the measurement head means that the beam of the scanning signal, propagating cone-like from the measurement head, is scattered more strongly on the container wall, the lower the level, so that low levels are not satisfactorily recorded here. An orientation of the measurement head deviating from the vertical means that, at low levels, the axis of the scanning signal cone can intersect the container wall, which is also undesirable.
The objective of the present invention is to provide a measurement head for a device for measuring the level of a product and such a device, which avoids the aforementioned alignment problems, and which is therefore easily installable by untrained personnel and suitable for obtaining more reliable measurement results.
The objective is realized, in the first place, by a measurement head with the features of claim 1 and, in the second place, by a level measurement device with the features of claim 4.
The measurement head according to the invention is equipped with at least one actuator to move the transmitter/receiver unit relative to its fastening element. This type of actuator can also always be operated after installation of the measurement head, if an unsatisfactory echo signal is received in a given position of the measurement head.
This type of actuator can be used to rotate or shift and/or tilt the measurement head; naturally, several actuators can also be combined, in order to move the measurement head in several degrees of freedom of rotation and/or translation.
The level measurement device according to the invention is equipped with such a measurement head and a control unit to operate its at least one actuator.
Automatic control of excitation of the actuator can occur according to various criteria. A first basic principle of control of the actuator is evaluation of the quality of the echo signal received in a given position by the control unit and operation of the actuator to vary the position of the measurement head when this quality is evaluated as deficient. Various criteria are conceivable for evaluation of the quality; for example, a signal can be considered deficient if its intensity or the signal/noise ratio falls short of a limiting value, or if the travel time of the echo signal lies outside of an admissible interval. The lower limit of this interval is generally defined by the aforementioned minimum spacing required for correct processing of the echo signal; the upper limit can be defined differently for different measurement positions. More precisely, the definition corresponds to the upper limit of travel time of an echo signal and the corresponding position of the measurement head with an empty container. This upper limit can be stipulated from the outside, for example, calculated beforehand as a function of the container geometry and entered in the control unit, but it is much more convenient and more flexible if the control unit defines the admissible interval for a position of the measurement head by means of a measurement conducted on the container in the empty state with the same position of the measurement head. This means it is assumed that the signal travel time in a non-empty container in each case must be shorter than the time measured in an empty container, so that the travel time measured in the empty container can be established as the upper limit of the interval.
The adjustability of the measurement head can also be used to perform measurements in different positions of the measurement head that correspond to different impingement regions of the signal cone on the product level. The variety of measured values so obtained can be used to record the surface profile of the product in the container and to calculate a quantity based on it for the volume of the product. In order to calculate the amount of product beneath the recorded level for such a calculation, a control unit requires stored data concerning the shape of the container. These data can be entered in the control unit beforehand, but the possibility of having the control unit generate these data itself by measurements conducted on the container in the empty state is particularly convenient and flexible.
It is also expedient if the control unit compares the measured values obtained for a variety of measurement head positions with an expected surface profile and discards the measured values whose deviation from the expected surface profile exceeds a limit value. This expected surface profile can have the shape of a debris cone or the like with a surface trend characteristic of the product being monitored. The level of this debris cone or the like in the container can be adjusted by means of the variety of obtained measured values, for example, by an optimization method, for example, according to the criterion of mean square error. If a measurement value is obtained in one position that deviates excessively from the value adapted to the expected debris cone or the like, it can be assumed that the measurement is disturbed in this position and the measurement value can be rejected. The deviation of the measured value obtained in an individual position from an expected surface-profile can also be used as a criterion for the quality of the echo signal at this position and cause the control unit to adjust a different position of the measurement head for continued measurement.
Moreover, the control unit can be designed so that excess deviations from the expected container profile recorded with time are used by the measurement head to generate warning messages. An operating person can therefore be simply informed about unusual changes within the measurement space.
Additional features and advantages of the invention are apparent from the following description of embodiment examples with reference to the figures. In the figures: